1. Field of the Invention
The present invention relates to a semiconductor device and, more particularly, to a bipolar power transistor.
2. Discussion of the Related Art
Power transistors must be able to withstand comparatively high power levels, and therefore comparatively high currents and voltages for sufficiently long periods of time. In order to increase the current value, emitter geometries having a high ratio between perimeter and area are provided, for example interdigital structures, so as to limit the phenomenon known as "emitter crowding" which, as is known, involves a crowding of the current in the peripheral zones of the emitter-base junction resulting in a degradation of the current gain.
A power transistor having an interdigital structure is formed from a plurality of identical elementary transistors having their collectors, as well as their emitters, connected together. Moreover, in a conventional structure of this type, the bases of the elementary transistors are connected together, as a result of which their relative base-emitter voltages are linked. In practice, the elementary transistors can only be considered to be roughly identical as they differ in general both with respect to their electrical properties and their operating temperatures. In particular, if there is an increase in the collector-emitter voltage, the current, rather than being distributing uniformly throughout all the elementary transistors, can be predominantly concentrated in certain transistors, as a result of which the power produced is substantially lower than the maximum power which the overall transistor could provide and regenerative processes leading to the breakdown of the transistor itself can occur. This breakdown mechanism, known as "forward secondary breakdown" (I s/b) is one of the major drawbacks of conventional power transistors.